1. Field
The present invention relates to safety devices for internal combustion engines and more particularly to an automatic fuel cut-off valve which remains open while the engine is running and closes when the engine is not running.
2. Prior art
It has been known to provide safety devices which respond to the presence or absence of engine vacuum and in some cases to provide an anti-back fire check valve and air bleed in such devices. Such devices are especially useful with 4-stroke cycle, multi-cylinder engines in which vacuum pulses from various cylinders are averaged in the engine manifold to provide an average negative pressure for operation of the device. Difficulty has been encountered in application of such prior devices to 2-stroke cycle engines and to some 4-stroke cycle engines. As a result it has become customary to provide manually operated, or electromagnetically operated fuel cut-off valves on apparatus such as motor cycles, garden tillers, grass mowers and the like. It is thought that these difficulties arise from the lack of a sustained negative pressure condition in the air intake system of the engine with which the device is used. For example, the air induction system of a single cylinder 4-stroke cycle engine may experience a subambient or negative pressure for approximately one-fourth of a cycle followed by ambient pressure for approximately three fourths of a cycle, depending somewhat upon valve timing and shape and size of induction passages. A 2-stroke cycle engine can produce alternations of sub-ambient and super-ambient pressures as the piston moves back and forth each cycle. Such variations of pressure can result in interruption of the fuel flow to the engine. While an anti-backfire valve, in theory, might be expected to function as a check valve for trapping subambient pressure, the massive movable elements in such valves may further contribute to the problem of pressure variation resulting in interrupted fuel flow, particularly where the engine is expected to operate over a range of speed from idle to wide open throttle.